High speed and multi-contact leds for data communication

ABSTRACT

An LED may have structures optimized for speed of operation of the LED. The LED may be a microLED. The LED may have a p− doped region with one or more quantum wells instead of an intrinsic region. The LED may have etched vias therethrough.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 62/936,767, filed on Nov. 18, 2019,and U.S. Provisional Patent Application No. 62/971,844, filed on Feb. 7,2020, the disclosures of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates generally to LEDs, and more particularlyto LEDs in an optical communication system.

Lasers tend to dominate optical communications on account of theirnarrow linewidth, single spatial mode output, and high-speedcharacteristics. The narrow linewidth of a laser means that high speedsignals can pass through dispersive mediums for long distances withoutpulse broadening. Long distance fiber optic links are frequently limitedby chromatic dispersion and thus a narrow linewidth laser may beessential for long distance fiber optic links. The single spatial modeof a laser is also relatively easy to couple to single mode fiber.

The stimulated emission of lasers may also allow for high modulationspeeds. Directly modulated optical links may be able to run at 25 Gb/seasily, and potentially carry 50 Gb/s of information using PAM4modulation.

However, use of lasers may present difficulties for opticalcommunications for very short distances, such as chip to chipcommunications.

BRIEF SUMMARY OF THE INVENTION

Some embodiments provide a LED configured for high speed operation. Insome embodiments the LED is used as part of a data communication system.In some embodiments the data communication system is an intra-chip,inter-chip, or intra-multi-chip module communication system. In someembodiments the LED is a microLED.

Some embodiments provide an optical communication system forcommunicating information provided by a processor to another area of theprocessor or another module in a multi-chip module, comprising: an LEDassociated with the processor; an LED driver to modulate output opticalpower of the LED, such that the LED will generate light based on dataprovided to the LED driver from the processor; a detector for performingoptical electrical conversion using the light, the detector for examplehaving an electrical output that is modulated by optical power incidenton the detector; and an optical waveguide optically coupling light fromthe LED to the detector; wherein the LED comprises: a p type layer; an ntype layer; and a lightly-doped recombination layer, the recombinationlayer including at least one quantum well between the p type layer andthe n type layer. Some embodiments provide an optical communicationsystem for communicating information provided by a first integratedcircuit (IC), for example a processor, to another area of that first IC,or to second IC in a multi-chip module, comprising: an LED associatedwith the first IC; an LED driver for activating the LED to generatelight based on data provided to the LED driver from the first IC; adetector for performing optical-electrical conversion using the light;and an optical waveguide optically coupling the LED and the detector;wherein the LED includes a plurality of etched vias. In someembodiments, the first and/or second IC is a processor.

These and other aspects of the invention are more fully comprehendedupon review of this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram showing an example of use of an LED, inaccordance with aspects of the invention.

FIG. 2A shows a typical p-i-n LED structure and FIG. 2B shows a banddiagram for the device of FIG. 2A.

FIG. 3A shows an optimized doping structure for an LED, in accordancewith aspects of the invention, and with FIG. 3B shows a band diagram forthe device of FIG. 3A.

FIG. 4 shows a microLED with etched vias, in accordance with aspects ofthe invention.

FIG. 5 includes a table indicating trade-offs in the design of themicroLED with typical parameters

DETAILED DESCRIPTION

FIG. 1 shows an example of use of an LED, which may be a microLED invarious embodiments, as variously discussed herein. In FIG. 1, a siliconprocessor 111 performs various operations on or with data. For example,the silicon processor may perform calculations on data, may performswitching functions, or may perform other functions. The siliconprocessor provides at least some of the data to an LED driver 113. TheLED driver activates the microLED 115 so as to optically provide the atleast some of the data from the processor, with the LED driver therebymodulating output optical power of the microLED so as to opticallyprovide the at least some of the data from the processor. Lightgenerated by the microLED is provided to an optical coupler 117, whichpasses the light into an optical propagation medium 119. The opticalpropagation medium, which may be for example a waveguide, may be used totransfer the light from one area of the silicon processor to anotherarea of the silicon processor in some embodiments. In other embodiments,the optical propagation medium may be used to transfer light from thesilicon processor to another silicon processor, or memory, or otherchip, for example in a multi-chip module (not shown in FIG. 1)(with theterm “chip” generally used interchangeably with “integrated circuit” or“IC,” unless the context clearly indicates otherwise). In doing so, theoptical propagation medium may transfer the light to another opticalcoupler 121, which in turn passes the light to a detector 123, forexample a photodiode, for optical-electrical conversion. The electricalsignal including the at least some of the data may be amplified by anamplifier 125, and provided to the silicon processor (or other chip in amulti-chip module). In some embodiments the microLEDs and detectors canbe coupled to waveguides individually, and/or in some embodiments theymay be coupled in parallel as arrays. The optical waveguides, inaddition to transferring the light and the data from one position toanother could also split the light into two or more outputs, allowingdata fanout. The optical waveguides or medium could also perform someswitching directing the output from one receiver to another receiver. Asa person skilled in the art would necessarily understand, the opticallinks may be duplex, so that when there are one or more links from afirst chip to a second chip, there may also be one or more links fromthe second chip to the first chip.

In some embodiments a microLED is distinguished from a semiconductorlaser (SL) as follows: (1) a microLED does not have an optical resonatorstructure; (2) the optical output from a microLED is almost completelyspontaneous emission, whereas the output from a SL is dominantlystimulated emission; (3) the optical output from a microLED istemporally and spatially incoherent, whereas the output from a SL hassignificant temporal and spatial coherence; (4) a microLED is designedto be driven down to a zero minimum current, whereas a SL is designed tobe driven down to a minimum threshold current, which is typically atleast 1 mA. In some embodiments a microLED is distinguished from astandard LED by (1) having an emitting region of less than 100 um×100 um(less than 10 um×10 um in some embodiments); (2) frequently havingpositive and negative contacts on top and bottom surfaces, whereas astandard LED typically has both positive and negative contacts on asingle surface; (3) typically being used in large arrays for display andinterconnect applications.

The microLEDs and detectors can be coupled to waveguides eitherindividually or in parallel as arrays. In some embodiments the microLEDsare microLEDs with structures optimized for speed, for example highmodulation speeds. In some embodiments the microLEDs are used forcoupling optical data into waveguides, in some embodiments providinghighly parallel communications between chips, for example on aninterposer or through a 3D optical structure, for example an opticalstructure that includes optical waveguides and/or free-space opticalpropagation with optical elements such as lenses and holograms. GaNbased microLEDs have been developed for display applications and apackaging ecosystem has been developed for mounting such devices onsilicon or polysilicon-on-glass backplanes. With relatively minormodifications, elements of such a packaging ecosystem may be used ininterconnecting ICs together for chip to chip communications.

In addition, for chip-to-chip communications, the distances are so shortthat material dispersion associated with the broad emission spectralwidth of the LED is not necessarily a problem. Simple calculationsindicate that, for a GaN LED with a center wavelength in the range of400 nm-450 nm and a 20 nm spectral width, if the LED is modulated at 4Gb/s and propagates through a doped SiO₂ waveguide or fiber, thewaveguide or fiber can be up to 5 meters long with a dispersion powerpenalty less than 2 dB. Since chip-to-chip communications inside of amulti-chip module (MCM) or across a PC board is typically less than tensof centimeters, the broad spectrum of an LED may not be an issue.Furthermore, one can even use highly multi-moded waveguides into whichit is relatively easy to couple output light of an LED. Since thedistances are short, modal dispersion of multimode waveguides may againnot be an issue. At a 4 Gb/s signal rate, even in a waveguide with a 10%core-cladding index step that has an NA of 0.67, the waveguide lengthcan be up to 85 cm with a small dispersion power penalty; smallercore-cladding index steps generally have longer reach. So broad spectrumLEDs and multi-mode waveguides are adequate in many embodiments forchip-to-chip communications.

Furthermore, in various embodiments the microLEDs are fabricated at verysmall sizes, with emitting area diameters of less than 2 um. Such asmall device has very high brightness and generally can be coupled to amultimode waveguide with high coupling efficiency. Though the output isgenerally Lambertian, with proper use of reflectors, microlenses in someembodiments, and imbedding the microLED in waveguide in someembodiments, coupling efficiencies can be 30% or more. MicroLEDstypically have high quantum efficiencies, similar to or even surpassingthat of lasers. Since over short distances one does not suffer muchwaveguide loss, even at the blue or green wavelengths, not much transmitpower is required and a small microLED can be sufficient, running atless than 10 uA in some embodiments.

The achievable modulation speed of a microLED in general is limited bythe carrier lifetime (and by capacitance if the microLED is too big),and generally cannot achieve the types of modulation speeds of lasers.However, clock speeds in microprocessors and logic seem to be reaching alimit of a few Gb/s. The input/output data of ICs is often sped up usingserializer/de-serializers (SERDES) to produce a smaller number of higherspeed lanes. For example, commercially available switch ICs maycurrently run at a few GHz clock speeds but communicate with 256 or 512lanes of 50 Gb/s or 100 Gb/s per lane. These SERDES consume a great dealof electrical power and can be eliminated if the switch IC instead usesa larger number of lower speed lanes. Optical interconnects allow muchhigher parallelism and higher total throughput, even at slower lanespeeds, by allowing the use of a much larger number of lanes.Nevertheless, getting LEDs to operate at modulation speeds that are ashigh as possible may be preferred.

Furthermore, microLEDs have substantial advantages over lasers in thatthey do not have a significant threshold current. Though quantumefficiency is a function of drive current, there is not a distinctthreshold level, and moreover, microLEDs can be run at far lowercurrents than lasers. Given their usefulness for displays, there is asubstantial infrastructure for mounting, connecting, and testingmicroLEDs on various substrates. And GaN microLEDs generally have farsuperior high temperature performance and reliability over semiconductorlasers.

Typically, a GaN microLED, optimized for display applications, comprisesa cylindrical or cylindrical-like structure with a p-i-n doping profile.The LED is turned on by forward biasing the diode and injectingelectrons from the n region and holes from the p region into the middleintrinsic region that contains InGaN quantum wells. A p contact is onone side of the structure, while an n contact is on the other. In manyapplications this cylinder is mounted onto a chip, where a “bottom” sideelectrically contacts the chip, and a “top” side contacts a common lead(e.g. a ground or power lead). The top side contact may be a transparentconductor such as indium-tin oxide (ITO). In microLEDs this “vertical”structure with contacts on the top and bottom of the LED is oftenpreferred, but there are also “lateral” structures where the n contactand p contact are located on the same surface. In any case, there is noneed to optimize these structures for speed because displays typicallyrun at 60 Hz or 120 Hz frame rate, not at Gb/s.

There are changes that one can make to optimize the structure for speed.In general, a microLED is limited by the capacitance of the LED and thecarrier recombination time. The capacitance forms an RC circuit with thedriver's output impedance and causes a roll-off at high frequencies. Thecarrier lifetime causes the LED to take time to turn off, as one has towait for the most of the minority injected carriers to recombine for thelight emission to significantly decrease, even after the electricalpulse has ended. Due to their small size, the capacitance of microLEDs,generally just a few femto-Farads, does not significantly limit devicemodulation speed; rather, modulation speed is generally limited bycarrier lifetime. Modulation speed can be increased by applying areverse bias to the microLED and shaping the applied pulse electricallyto pull the carriers out, but structural changes can also be made to thedevice to improve modulation speed.

A typical LED structure is comprised of a p type region, an “active”region where carriers recombine and light is emitted, and a n typeregion. There are numerous different LED structures that differ in thestructure of the active region. In some embodiments, the active regioncontains one or more quantum wells (QWs)

Generally the speed of a microLED increases with current level. Thereare three ways that carriers can recombine in LEDs. At low currentlevels, the recombination is mediated by traps (known as SRHrecombination). At higher current densities these traps become saturatedand the quantum efficiency of the LED improves, as radiativerecombination dominates. This radiative recombination rate speeds up asthe carrier density increases, increasing the radiative efficiency andreducing the carrier lifetime. Thus, the harder (e.g. greater currentdensities) the microLEDs are driven, the faster they operate. At highcurrent densities, nonlinear non-radiative mechanisms such as Augerrecombination further reduces carrier lifetime, but these non-radiativemechanisms also reduce the radiative quantum efficiency. For a fastmicroLED with a small diameter to increase the current density at agiven current, the traps are relatively unimportant as they aresaturated, and the relative significance of nonlinear nonradiativerecombination versus radiative recombination rate determines the quantumefficiency.

Some embodiments utilize a p, p−, n structure where the “intrinsicregion” is doped p− type at reasonable levels—10{circumflex over( )}16/cm{circumflex over ( )}3 to 10{circumflex over( )}17/cm{circumflex over ( )}3 in some embodiments. In some embodimentscompared to a p-i-n structure, this results in a much narrower depletionwidth in the p− region. Electrons, which have high mobility, areinjected into the p− depletion region that already has a high density ofholes. Since the carrier recombination time is a function of the carrierdensity, the speed of the device increases as the depletion widthdecreases. The carrier recombination time is also a function of theproduct of the electron and hole densities, and p− doping in thedepletion region increases the density of holes, thus increasing therecombination rate and decreasing the recombination time. The narrowerdepletion region may also have the undesirable effect of increasing themicroLED's capacitance, but this may not be important for structureswith very small diameters since the RC time constant will still be muchsmaller than the recombination time.

FIG. 2A shows a typical p-i-n LED structure and FIG. 3A shows anoptimized doping structure, with FIGS. 2B and 3B also showing associatedband diagrams for the devices of FIGS. 2A and 3A, respectively. Thedevice of FIG. 2A has a p doped GaN layer 211 and an n doped GaN layer215 sandwiching an intrinsic region 213 having InGaN quantum wells. Thedevice of FIG. 3A also has a p doped GaN layer 251 and an n doped GaNlayer 255 sandwiching an intermediate region. The intermediate region inthe device of FIG. 3A, however, is doped p− type, and also containsInGaN quantum wells. In some embodiments the quantum wells are locatedphysically closer to the p doped GaN layer than the n doped GaN layer.

The band diagram of FIG. 2B shows a conduction band 231 above a valenceband 233, across an n region 221, an intrinsic/depletion region 223, anda p region 125. A bandgap between the conduction band and the valenceband is generally constant across the regions, with energy levelsgenerally increasing in the intrinsic/depletion region between the n andp regions, such that energy levels are higher in the p region than the nregion. Electrons are injected 235 from the n region into theintrinsic/depletion region, holes are injected 237 from the p regioninto the intrinsic/depletion region, in which recombination 239 occurs.

The band diagram of FIG. 3B also shows a conduction band 261 above avalence band 263, across an n region 221, a depletion/p− region 265 a,b,respectively, and a p region 225. Compared to the band diagram of FIG.2B, it may be seen in FIG. 3B that the depletion/p− region replaces theintrinsic/depletion region, with the depletion region 265 adjacent the nregion and the p− region 265 b adjacent the p region 225. A bandgapbetween the conduction band and the valence band is generally constantacross the regions, with energy levels generally increasing in thedepletion/p− region, primarily in the depletion region, are higher inthe p region 225 than the n region 221.

FIG. 3B, compared to FIG. 2A, also shows electron injection 275 over amuch thinner depletion region into the p− region and the recombinationis generally occurring there. In the GaN material system, the increasein background doping may decrease the radiative recombination time by atleast an order of magnitude or a few orders of magnitude.

Though FIG. 3A shows a p, p−, n structure, one could also dope thequantum wells n type rather than p type. This also increases the speedthe microLED compared to a p-i-n structure. The advantage of n dopingversus p doping is that n doping does not increase defects and would notreduce radiative efficiency. The doping level can also be furtherincreased to reduce the carrier recombination time at the price of ahigher capacitance.

Some embodiments include further modifications to the doped structure ofFIG. 3B that may further improve the performance. For example, someembodiments use an AlGaN barrier on the n region to further enhanceinjection of carriers into the p− doped recombination region and preventhole injection into the n type region. Some embodiments optimize InGaNquantum wells in the p− region in terms of number, width, and strain todecrease recombination time. For example, a lower In concentration thatpushes the wavelength to shorter wavelengths also increases the speed.Thus, microLEDs with wavelengths between 380 nm to 430 nm may beintrinsically faster than those at longer wavelengths. Fewer quantumwells also increase the carrier density in the quantum well for a givencurrent. The carrier recombination time decreases faster as carrierdensity increases. So in some embodiments the microLED has only one or afew quantum wells. In some embodiments the quantum well width is alsomade smaller. A smaller quantum well width brings the electrons andholes closer together, with an increased overlap integral and reducedradiative recombination time. Some embodiments use an appropriate GaNsubstrate for growth to reduce the built-in electric field in thequantum wells, increasing the overlap integral between the electrons andholes and thus further reducing carrier recombination time. One can alsoreduce the built-in field by going to a smaller mole fraction of indium,once again getting faster response in the short wavelength range. Asmaller indium concentration also lowers the Auger recombination rate,increasing the quantum efficiency of the LED.

In some embodiments the structure optimized for high speed operation hasa small size, with a diameter of less than about two microns to increasethe current density and the carrier density. In some embodiments thestructure optimized for high speed operation has few quantum wells,perhaps only one, so that at a given current density the carrier densityis maximized. In some embodiments the indium concentration of thequantum well is low, thus the microLED would emit at shorter wavelength,for example blue or ultraviolet wavelengths, as a smaller indiumconcentration would give a lower piezo-electric field that increases thehole-electron wavefunction overlap integral and thus increasesrecombination rate. In some embodiments the quantum well is small,typically 2 nm or less, to increase the overlap between electrons andholes. In some embodiments the quantum wells are doped either p type orn type to increase the background carrier density.

Table I of FIG. 5 describes the trade-offs in the design of the microLEDwith typical parameters.

In general, higher doping also decreases the non-radiative recombinationtime. This further shortens the carrier lifetime and increases themodulation speed, but with a penalty of reduced quantum efficiency. Onceagain, in these very short distance applications where there is littlewaveguide propagation loss, quantum efficiency be less important thanmodulation speed. Fundamentally, there is a trade-off between quantumefficiency and modulation speed: the overall LED recombination rate canbe increased by increasing the nonradiative recombination rate, which inturn reduces the quantum efficiency. Accordingly, in some embodimentsthe speed an LED is increased, dramatically in some embodiments, at theprice of a lower quantum efficiency.

Fast recombination centers can be induced in the LED by a number ofprocesses. These include a lower temperature growth of the crystal inthe intrinsic region, proton implantation, deliberately induced defectdensity using dislocations in the crystal lattice, roughening the etchedsurface, or increasing the exposed surface area through othertechniques.

Generally, smaller microLEDs tend to have lower quantum efficienciesbecause carriers diffuse and recombine at the etched outer surface. Thisreduces the carrier lifetime, and therefore also increases the speed ofthe microLED. This effect can be increased by etching structural holesor vias in the structure that expose more area in the via sidewalls,creating more recombination centers. FIG. 4 shows a microLED with etchedvias. The example of FIG. 4 shows a microLED with a generallycylindrical shape, extending between a circular base 419 to a circulartop 417. The microLED may include a base layer 413 (which may be, e.g.,an n GaN layer) extending upward from the circular base and a top layer411 (which may be, e.g., a p GaN layer) extending downward from thecircular top. A middle layer 415 is between the base layer and the toplayer, and the middle layer may provide, as one would understand, anintrinsic depletion region or a p− depletion region.

The microLED of FIG. 4 also includes etched vias, for example etched via421, extending from the circular top to the circular bottom. The etchedvias therefore provide apertures through the microLED, from the topsurface to the bottom surface. In FIG. 4, the etched vias are ofcircular cross-section, thereby forming a cylindrical via, with the viasgenerally arranged in a square or diamond pattern. The etched vias mayinduce non-radiative recombination at the exposed surface to reduce thecarrier lifetime and therefore increase the speed. This may provide amore controllable method than proton implant or lower temperaturegrowth. In this case when the device is etched to form the microLED,various structures can be used to increase the surface area. Theseinclude etching multiple pillars in some embodiments, and/or etchedvias, as the figure shows. Other shapes such as stars or roughened edgesmay also or instead be used in some embodiments.

Although the invention has been discussed with respect to variousembodiments, it should be recognized that the invention comprises thenovel and non-obvious claims supported by this disclosure.

What is claimed is:
 1. An optical communication system for communicatinginformation provided by a processor to another area of the processor oranother chip in a multi-chip module, comprising: an LED associated withthe processor; an LED driver to modulate optical output power of theLED, such that the LED will generate light based on data provided to theLED driver from the processor; a detector for performingoptical-electrical conversion using the light; and an optical waveguideoptically coupling light from the LED to the detector; wherein the LEDcomprises: a p type layer; an n type layer; a lightly dopedrecombination layer, the recombination layer including at least onequantum well between the p type layer and the n type layer.
 2. Thesystem of claim 1, wherein doping of the lightly doped recombinationlayer comprises p− doping.
 3. The system of claim 2, wherein the p−doping is in the range of 10¹⁶/cm³ to 10¹⁷/cm³.
 4. The system of claim1, wherein doping of the lightly doped recombination layer comprises n−doping.
 5. The system of claim 1, wherein the LED is a microLED.
 6. Thesystem of claim 1, wherein the p type layer and the n type layer arecomprised of GaN and the at least one quantum well comprises InGaN. 7.The system of claim 1, further comprising: a further LED associated withthe other area of the processor or other chip in the multi-chip module;a further LED driver to modulate optical output power of the furtherLED, such that the further LED will generate light based on dataprovided to the further LED driver from the other area of the processoror other chip in the multi-chip module; and a further detector forperforming optical-electrical conversion using the light from thefurther LED; wherein the further LED comprises: a p type layer; an ntype layer; a lightly doped recombination layer, the recombination layerincluding at least one quantum well between the p type layer and the ntype layer.
 8. The system of claim 7, wherein the optical waveguideoptically couples light from the further LED to the further detector. 9.An optical communication system for communicating information providedby a processor to another area of the processor or another chip in amulti-chip module, comprising: an LED associated with the processor; anLED driver for activating the LED to generate light based on dataprovided to the LED driver from the processor; a detector for performingoptical-electrical conversion using the light; and an optical waveguideoptically coupling the LED and the detector; wherein the LED includes aplurality of etched vias.
 10. The optical communication system of claim9, wherein the LED is a microLED.
 11. The optical communication systemof claim 9, further comprising: a further LED associated with the otherarea of the processor or other chip in the multi-chip package; a furtherLED driver for activating the further LED to generate light based ondata provided to the further LED driver from the other area of theprocessor or other chip in the multi-chip package; and a furtherdetector for performing optical-electrical conversion using the lightfrom the further LED; wherein the further LED includes a plurality ofetched vias.